I will study the organization and expression of the histone genes of the mouse. Several sets of histone genes have been isolated from a mouse gene libraty. The expression of the genes will be studied in synchronized cultured cells; myeloma cells synchronized by isoleucine starvation, 3T3 cells synchronized by serum starvation and CHO cells synchronized by mitotic shake-off. We will determine whether the genes are controlled at the level of transcription, processing of nuclear RNA or stability of the mRNA. Using isolated nuclei active in RNA synthesis we will study the synthesis of histone mRNA in nuclei from different stages of the cell cycles. Factors involved in control of the synthesis of histone mRNA will be detected in extracts of S phase cells. These factors will be detected by activation of histone mRNA synthesis in the stage nuclei inactive in histone RNA synthesis.